1. Field of the Invention
This invention relates to a method for curing a polyurethane coating, and more particularly to a curing method suitable for the polyurethane coating to be formed on the surface of a weather-strip destined to be slid against a glass in an automobile.
2. Description of the Prior Art
On the surface of a weather-strip destined to be slid against a glass in an automobile, a coating 2 of polyurethane paint is superposed as illustrated in FIG. 2 with a view to lowering the sliding resistance and enhancing the resistance to wear. The polyurethane paint used for this purpose is of one-component type or two-component type invariably containing isocyanate prepolymer. The coating formed of either type of polyurethane paint cures slowly. When the coating is left curing at room temperature or at a temperature not exceeding 100.degree. C., the time required for the coating to be cured to a degree enough to acquire required coating properties is so long as to impair productivity of the weather-strip.
In the case of a weather-strip made of EPDM (ethylenepropylene-diene terpolymer), for example, the practice of applying a polyurethane coating on the surface of the weather strip exposed to sliding against the glass and subsequently allowing the applied coating to stand in a heating atmosphere of about 150.degree. C. for 6 to 10 minutes to cure has prevailed to date. Recently, adoption of such a thermoplasitc elastomer as PVC (polyvinyl chloride) or PO (polyolefin) of lower resistance to heat as the substrate for the weather-strip has come to be studied. Consequently, the desirability of developing a method capable of curing the polyurethane coating in a heating atmosphere not exceeding 100.degree. C. in a brief period has been finding growing recognition.
The inventors found the specification of Japanese Patent Publication No. SHO 53(1978)-19,038 to disclose a method for curing a polyurethane coating by treating the coating with a catalyst vapor for a prescribed period. They have studied this method in an attempt to apply this method to the curing of the polyurethane coating on the leather strip, to learn that this method raises the following problems.
(a) This method does not permit use of any catalyst which is not easily vaporized. This fact imposes a limit on the selection of an optimum catalyst for the polyurethane coating.
(b) Where the production of weather-strip is to be carried out continuously, an attempt to incorporate the step for vapor treatment into a closed system proves to be difficult. Inclusion of this vapor treatment, therefore, is detrimental to the safety of the shop environment.
(c) Since the concentration of the catalyst vapor is not allowed to increase appreciably, the time required for the catalyst vapor to pass into the coating is too long to permit a sufficient reduction in the time for curing the coating.